Technical Field
The disclosure relates to an electrical power supply circuit and to a method of supplying electrical power. The disclosure has particular utility to applications in which a source voltage is stepped up/down to provide an output voltage higher/lower than the source voltage.
Background Description of the Related Art
Some components of battery powered devices use voltages higher than a maximum voltage that can be supplied by the battery. For example, mobile communication devices typically have lithium ion batteries able to supply a maximum voltage between 4.2V and 3V or down to 2.5V for a low cut-off one. As the battery is drained, the maximum voltage it is able to supply decreases, meaning that the supply voltage can often decrease to 2.5V. One component that uses a higher supply voltage is a radio frequency (RF) power amplifier (PA), which uses approximately 3.4V in a Global System for Mobile Communications (GSM) mobile communication device, and as much as 4.5V when Multi Mode Multi Band (MMMB) is employed.
In order to deal with this, many mobile communication devices incorporate a step-up converter. However, step-up converters have some principal drawbacks that limit their ability to supply an RF PA directly, the main one of which is that their output has high ripple, which can cause degradation of the RF signal in the RF PA due to an unwanted inter-modulation. Another disadvantage of step-up converters is the low bandwidth of their control loop, which is a limiting factor in terms of envelope tracking technique use.
In some devices, a cascade of a step-up converter and a step-down converter is provided. The RF PA is supplied by the step-down converter, which has a low ripple and a high speed regulation loop. The step-up converter generates a boosted battery voltage to supply the step-down converter. Unfortunately, if the step-down converter is always supplied by the step-up converter, there can be inefficiency due to losses in the step-up converter, particularly when the battery voltage would be sufficiently high without boosting. It has therefore been proposed to switch between the battery supplying the step-down converter directly and the step-down converter being supplied by the step-up converter. This offers an improvement, but the switching cannot be carried out during operation of the RF PA, as it can cause a large jump in the output voltage of the step-down converter. The right supply for the step-down converter can be selected in advance, before operation of the RF PA starts. However, this requires knowledge of the likely battery voltage and expected maximum output voltage, which is not straightforward and generally requires information from a digital RF control chip.